On-line monitoring of cyanide concentration via a gas membrane system in extractive metallurgical processes

The principle of molecular diffusion of gaseous HCN across a tubular microporous hydrophobic PTFE membrane directly immersed in an alkaline solution or in an aqueous mineral suspension has been applied to the design of an on-line sensing system for cyanide. It offers an efficient detection of “available” cyanide and does not require acidification and solution sampling in the reactor. Gaseous HCN diffused through the membrane is dissolved by a sodium hydroxide carder solution which is then submitted to a spectrophotometric analysis by the pyridine-barbituric acid method. The problems related to filtering of sludges are therefore overcome, and many chemical interferences can be eliminated. Complexation reactions of cyanide by metallic cations and the cyanide-consuming properties of sulphide minerals and ores were studied. This method allows the determination of concentrations down to ca. 6 × 10^?7 mol l^?1 HCN + CN^? in the reactor at Ph 10 with a 120-cm membrane tube and a carrier solution flow-rate of 8 cm³ min^?1.